1. Field of the Invention
The present invention relates to an inverter and a method for driving the same, and more specifically to a circuit and a method for driving a piezoelectric transformer to efficiently and stably light a plurality of cold cathode tubes used as a load and to suppress generation of an audible sound.
2. Description of Related Art
In general, a piezoelectric transformer is a device in which an alternating voltage of a resonance frequency is applied to a primary electrode to cause a mechanical vibration by resonance, and a stepped-up voltage is picked up from a secondary electrode by the mechanical vibration. This piezoelectric transformer has a feature that it can be easily scaled down and thinned in comparison with an electromagnetic transformer, and therefore is attractive for use as a backlight power source for a liquid crystal display. The piezoelectric transformer has a characteristic in which the step-up ratio varies upon the impedance of the load as shown in the step-up characteristics shown in FIG. 9. On the other hand, the cold cathode tube has a characteristic in which a high voltage is required to cause a discharge to start, as shown in FIG. 10. At this time, the impedance of the cold cathode tube viewed from the piezoelectric transformer becomes very large. Thereafter, if a tube current starts to flow as the result of the start of the discharge, the impedance of the cold cathode tube lowers, so that the voltage across the cold cathode tube abruptly drops. However, if the tube current reaches some degree, the drop of the voltage becomes slow. Accordingly, considering the cold cathode tube as a load, the piezoelectric transformer is a device having the characteristics adapted to driving of the cold cathode tube, in which at the time of starting the lighting, a high step-up ratio can be obtained, and if the tube current flows and the lighting becomes stable, the step-up ratio lowers.
In the prior art, as a circuit for driving this type of piezoelectric transformer, a circuit has been known which lights a plurality of cold cathode tube in series as a load for one piezoelectric transformer, as disclosed in Japanese Patent Application Pre-examination Publication No. JP-A-08-045679. This circuit is shown in FIG. 11. This circuit includes an inverter 1 for converting a DC voltage into an AC voltage of a high frequency, a DC power source E for supplying an electric power to the inverter 1, a piezoelectric transformer 2 having primary electrodes receiving the AC voltage and configured that an AC voltage Vo stepped up by a piezoelectric effect is obtained from secondary electrodes 2b, a plurality of series-connected cold cathode tubes 3 connected to the secondary electrodes 2b of the piezoelectric transformer 2 as a load, a resistor R for detecting a tube current Io, a diode D for rectifying the AC voltage, an integrator 4 composed of a smoothing circuit, and a voltage-to-frequency (V-F) converter 5 for controlling a resonance frequency f of the inverter 1 on the basis of the voltage smoothened by the integrator 4.
Now, an operation of the prior art example shown in FIG. 11 will be described. The DC voltage supplied from the DC power source E is converted to the AC voltage by the inverter 1. If the obtained AC voltage having the frequency f is applied to the primary electrodes 2a of the piezoelectric transformer 2, the AC voltage Vo of the frequency f stepped up by the piezoelectric effect is outputted from the secondary electrodes 2b. This AC voltage Vo of the frequency f is supplied to the series-connected cold cathode tubes 3 as the load for the piezoelectric transformer 2. The tube current Io flowing in the load is converted to an AC voltage by the resistor R, and rectified by the diode D, and then, is converted to a smoothened voltage by the integrator 4. On the basis of the voltage smoothened by the integrator 4, the V-F converter 5 controls the resonance frequency f of the inverter 1 and therefore controls the step-up ratio of the piezoelectric transformer, thereby to supply a predetermined tube current Io to the load. By the cold cathode tube driving circuit having the above mentioned circuit construction, a plurality of series-connected cold cathode tubes 3 are lighted in series.
The circuit shown in FIG. 12 includes a piezoelectric transformer 2, a support mechanism 10 for mechanically supporting this piezoelectric transformer 2, a driving circuit 7 for converting a DC voltage into two half-wave sine wave voltages generated alternately, to alternately supply the half-wave sine wave voltages to a pair of primary electrodes 2a of the piezoelectric transformer 2, a DC power supply E for supplying an electric power to the driving circuit 7, a frequency sweep oscillator 6 for controlling the frequency fk of the two half-wave sine wave voltages of the driving circuit 7, a cold cathode tube 3 connected to secondary electrodes 2b of the piezoelectric transformer 2, a load current comparing circuit 8 for comparing the tube current Io flowing in the cold cathode tube 3 with a reference value, for the purpose of controlling a frequency sweep direction of the frequency sweep oscillator 6, and a light adjusting circuit 9 for outputting a light adjusting circuit for driving the piezoelectric transformer 2 in a time-divided manner, to the frequency sweep oscillator 6 and the drive circuit 7.
Now, an operation of the example shown in FIG. 12 will be described. The DC voltage supplied from the DC power source E is converted by the driving circuit 7 into the two half-wave sine wave voltages, which are applied to the pair of primary electrodes 2a of the piezoelectric transformer 2. Thus, the sine wave voltage Vo of a high voltage, stepped up by the piezoelectric effect, is outputted from the secondary electrodes 2b. This sine wave voltage Vo of the high voltage is supplied to the cold cathode tube 3 connected as the load for the piezoelectric transformer 2. The tube current Io flowing in the load is supplied to the load current comparing circuit 8, and is compared with the reference value the inside of the load current comparing circuit 8. If the tube current is smaller than the reference value, the load current comparing circuit 8 outputs a signal for causing the frequency sweep oscillator 6 to lower the frequency fk of the half-wave sine wave voltages outputted from the driving circuit 7. If the tube current is larger than the reference value, the load current comparing circuit 8 outputs a signal for elevating the frequency. The frequency sweep oscillator 6 outputs the drive signal of the frequency fk on the basis of the signal from the load current comparing circuit 8, to the drive circuit 7. The drive circuit 7 drives the piezoelectric transformer 2 by the two half-wave sine wave voltages of the frequency fk. Thus, it is possible to supply a predetermined tube current Io to the load by controlling the drive frequency fk of the piezoelectric transformer 2 to change the step-up ratio of the piezoelectric transformer.
Next, the light adjusting section will be described. The light adjusting circuit 9 oscillates at a frequency fc which is sufficiently lower than the frequency fk of the piezoelectric transformer 2 and which does not give a flicker to eyes, and outputs a binary light adjusting signal having a duty ratio varying upon an inputted light adjusting voltage, to the frequency sweep oscillator 6 and the drive circuit 7. If this light adjusting signal is at a high level, the drive circuit 7 stops the driving of the piezoelectric transformer 2, so as to stop the tube current Io flowing in the load. During the period in which the tube current Io is stopped, the frequency sweep oscillator 6 operates to maintain the drive frequency fk just before the tube current Io is stopped, in order to prevent the drive frequency fk from changing towards a low value by action of the load current comparing circuit 8. This can ensure that when the light adjusting signal is brought to a low level so that the drive circuit 7 restarts to drive the piezoelectric transformer 2 and therefore the tube current Io starts to flow, the tube current does not change.
Operating waveforms in the example shown in FIG. 12 is shown in FIG. 13. The light adjusting signal which is the output of the light adjusting circuit 9 alternately becomes the high level and the low level at the period of the frequency fc. During the high level period, since the outputting of the two half-wave sine wave voltages of the frequency fk, which is the driving voltage of the piezoelectric transformer 2, is stopped, the sine wave voltage Vo of the high voltage and the frequency fk is not outputted from the piezoelectric transformer 2, and therefore, the tube current Io flowing in the load is stopped. In order to maximize the brightness of the cold cathode tube 3, the light adjusting signal is ceaselessly maintained at the low level, and in order to lower the brightness, the high level period of the light adjusting signal is elongated. Thus, the light adjustment is performed by a pulse width modulation (PWM) in which the time period of causing the tube current Io to flow in the cold cathode tube 3 is changed by adjusting the duty ratio of the light adjusting signal which is the output of the light adjusting circuit 9.
In FIG. 12, a plurality of cold cathode tubes 3 used as the load are lighted in parallel by using a plurality of piezoelectric transformer inverters having the above mentioned construction.
However, the prior art disclosed in JP-A-08-034679 shown in FIG. 11 and the example and shown in FIG. 12 have the following problems.
A first problems is that when a plurality of cold cathode tubes 3 are lighted in series as the load for the piezoelectric transformer 2 as shown in FIG. 11, the number of the cold cathode tubes which can be connected in series is limited. Reviewing the characteristics diagram of the mechanical vibration speed "vm" to the output power Pout of the piezoelectric transformer 2 shown in FIG. 14A, it would be seen that the output power Pout of the piezoelectric transformer 2 is substantially in proportion to the mechanical vibration speed "vm". In order to light a plurality of series-connected cold cathode tubes 3, there is required the electric power obtained by multiplying the electric power necessary to drive one cold cathode tube 3 by the number of series-connected cold cathode tubes 3. In order to obtain a large power from the piezoelectric transformer 2, it is necessary to make the mechanical vibration speed "vm" large. However, if the mechanical vibration speed "vm" is made large, the temperature elevation .DELTA.T increases from ambient temperature as shown in FIG. 14B, and the efficiency .eta. lowers as shown in FIG. 14C. In addition, if the mechanical vibration speed "vm" reaches "vm1", the temperature elevation .DELTA.T starts to abruptly increase, and the efficiency .eta. abruptly drops. In other words, in a region higher than the point .DELTA.T1 where the temperature elevation .DELTA.T starts to abruptly increase, and in a region lower than a point .eta.1 where the efficiency .eta. abruptly drops, it cannot be used as a high efficient piezoelectric transformer inverter. Therefore, the output power Pout1 having the temperature elevation .DELTA.T1 and the efficiency .eta.1 can be said to be a limit value of the output power Pout of the piezoelectric transformer 2 in the high efficient piezoelectric transformer inverter. For example, if the cold cathode tube 3 having the tube length 360 mm and tube diameter 3.phi. is driven with the tube current Io=4.4 mArms by using the piezoelectric transformer 2 having a piezoelectric element of 42 mm in length, 10 mm in width and 1 mm in thickness, the output power limit value Pout1 is 7 W. Since the output power Pout of the piezoelectric transformer 2 in the case of driving one cold cathode tube 2 is 3.5 W, the number of cold cathode tubes which can be series-connected to the piezoelectric transformer 2 is limited to two. As mentioned above, with enlargement of the screen size of the liquid crystal display panel, a plurality of long cold cathode tubes are used in a multi-tube back light, the degree of freedom in the series connection of cold cathode tubes is restricted. Actually, there is no room in selection.
A second problem is that when a plurality of cold cathode tubes are lighted in series as the load for the piezoelectric transformer 2 as shown in FIG. 11, the brightness is uneven. The reason for this is that if a conductive reflecting plate exists near to the cold cathode tubes, a floating capacitance is formed between the cold cathode tubes and the reflecting plate. The current flows into the floating capacitance, with the result that at a high voltage electrode side of the cold cathode tube 3, the tube current value is large and the brightness is high, but at a low voltage electrode side of the cold cathode tube 3, the tube current value is small and the brightness is low. The longer the tube length of the cold cathode tube 3 is, the larger the formed floating capacitance becomes. Therefore, this phenomenon becomes remarkable if a plurality of cold cathode tubes are connected in series.
A third problem is that when a plurality of cold cathode tubes 3 are lighted in parallel as the load for the piezoelectric transformer 2 as shown in FIG. 12, since the respective drive frequencies fk and/or the respective light adjusting frequencies fc are asynchronous, a flickering occurs because of mutual interference of the plurality of cold cathode tubes 3. Since each of the piezoelectric transformer inverters has the frequency sweep oscillator 6, even if variation exists in the step-up characteristics of the piezoelectric transformer 2 shown in FIG. 9 and the voltage-current characteristics of the cold cathode tube 3 shown in FIG. 10, all of the tube currents Io flowing in the respective cold cathode tubes are controlled to a constant value by changing the drive frequencies fk. Therefore, since the drive frequency fk becomes different from one piezoelectric transformer inverter to another, the cold cathode tubes 3 are mutually coupled through the floating capacitance formed between the cold cathode tubes, so that the tube currents Io is amplitude-modulated as shown in FIG. 15. In this condition, if the brightness is lowered by the PWM light adjustment, a flicker occurs when the brightness becomes lower than a certain value. In addition, if the respective light adjusting frequencies fc are out of synchronism with each other, the high voltage at the driving starting time and the driving stopping time of the piezoelectric transformer 2 change the tube current value flowing in another cold cathode tube 3 by means of the mutual coupling of the cold cathode tubes through the floating capacitance. As a result, the operation of the circuit such as the load current comparing circuit 8 and the frequency sweep oscillator 6 for controlling the drive frequency fk in order to maintain the tube current Io at a constant value, is adversely influenced, so that he operation of the inverter becomes unstable.
A fourth problem is that the generation of the audible sound increases when a plurality of long cold cathode tubes are lighted in series as the load for the piezoelectric transformer 2. When the piezoelectric transformer 2 is controlled in the PWM manner for light adjustment, the drive voltage of the piezoelectric transformer 2 becomes a burst form as the drive voltage Vd shown in FIG. 13, and therefore, at the driving starting time and at the driving stopping time, the drive frequency fk includes harmonic components of the light adjusting frequency Fc. If the piezoelectric transformer 2 is driven with the drive voltage Vd of the drive frequency fk including the harmonic components, the vibrating condition of the piezoelectric transformer 2 is momentarily disturbed at the driving starting time and at the driving stopping time, so that this vibration is propagated to the support mechanism 10 of the piezoelectric transformer 2. This becomes the vibration of the piezoelectric transformer 2 and the support mechanism 10, so that the audible sound occurs. Even if the generation of the audible sound is low in the case of driving a long cold cathode tube 3, when a plurality of long cold cathode tubes 3 are driven in series, since the discharge start voltage and the tube voltage for the long cold cathode tubes to be lighted become high, the mechanical vibration speed "vm" of the piezoelectric transformer 2 increases, and therefore, the audible sound is apt to easily occur.